Sway-Rocking Spring System Applicable to Short-Rigid Monopile Foundations

Sway-Rocking (SR) spring system is a foundation modelling method in which a 2 × 2 spring stiffness matrix represents the foundation stiffness at seabed level. Since each individual element of the spring stiffness matrix is a function of a few physical properties of the soil and pile, it is the simplest and computationally cheapest method to estimate the deformation at the seabed level and the natural frequency of an offshore wind turbine structure. Formulae currently available are only applicable to conventional long-flexible piles or perfectly rigid foundations such as footings or caissons. This study aims to extend the application of the SR spring system to a so-called short-rigid pile range, which lays between the two ranges: long-flexible and perfectly rigid. Monopiles, the most commonly selected type of offshore wind foundations, are categorized in the short-rigid pile range. By conducting three-dimensional finite element analyses, the short-rigid pile behaviour is investigated. Based on the results, a new set of formulae of the SR spring stiffness, which is applicable to a wide range (long-flexible, short-rigid and perfectly rigid) of piles, is proposed. The new formulae also improve the accuracy of frequency estimation for practical use cases.